head to head
Neither is simply harder - they are hard in different places. Call of Juarez demands more execution; Grand Theft Auto: San Andreas demands more optimization.

Execution · Practiced Mastery
A dual-protagonist western shooter testing aim and learned stealth routes through scripted levels.

Optimization · Practiced Mastery
A sprawling open-world crime epic with RPG-style character stats across a whole state.
Call of Juarez leans Execution (Practiced Mastery); Grand Theft Auto: San Andreas leans Optimization (Practiced Mastery).
Call of Juarez leans clearly harder on execution.
Call of Juarez
Gunplay and whip platforming need steady aim and timing
Grand Theft Auto: San Andreas
Flying school and drive-bys need decent hands
Call of Juarez leans a little harder on reads, though the gap is small.
Call of Juarez
Campaign enemies are scripted; multiplayer is an afterthought
Grand Theft Auto: San Andreas
Gang AI is scripted, never adaptive
Same score on variance - which means the number cannot separate them. The difference is in what each one asks of you.
Call of Juarez
Stealth patrols are unknown first time through, then fixed
Grand Theft Auto: San Andreas
Territory wars and traffic add light randomness
Grand Theft Auto: San Andreas leans clearly harder on optimization.
Call of Juarez
Learning each chapter's intended approach for both characters
Grand Theft Auto: San Andreas
Stats, gym, weapon skill, and routes reward systematic play
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.